Outgassing of the crystal resonator package limits the long term stability of crystal resonators. In the past, ceramic, metal, and glass packages have been used as crystal resonator packages for resonators of good long term frequency stability. Although ceramic packages have the best outgassing properties, the difficulty with the ceramic packaging is that it is very expensive to implement, and plagued with yield problems due to difficulties related to the ceramic package. The problems include the difficulty of maintaining the flatness of the ceramic parts during metallization and firing, breakage when the sealing pressure is applied to parts that are not sufficiently flat, delamination of the metallizations, and leaks due to various causes, and high manufacturing costs.
Metal packages have been made of nickel plated copper, or nickel, or kovar (which is an iron-nickel-cobalt alloy), or copper clad kovar, or gold plated nickel clad copper. The bases of metal packages, where the electrical leads enter the package, have been made of kovar, as kovar is a standard material for glass to metal seals. Aluminum has been thought not to be suitable for packages because aluminum has relatively poor mechanical properties, aluminum is relatively difficult to weld, and ordinary aluminum is not a good vacuum material due to the outgassing of impurities in the material. Outgassing is highly undesirable because the contaminants produced by the outgassing will adsorb onto the resonator surfaces and thereby degrade the resonator's stability. Lately, however, pure aluminum has been found to be an outstanding vacuum material. In fact, vacuum systems containing vacuum chambers made of pure aluminum, with smooth and properly cleaned inner surfaces, have achieved 10.sup.-13 torr pressures. These are among the lowest pressures ever achieved on earth.